Require filesystem backends to implement directory walks (#39749)

## What changed

- Make `ExecutorFileSystem::walk` a required backend operation instead of
  providing a fallback built from directory reads and metadata requests.
- Implement bounded local walks on a blocking task with cancellation, symlink
  cycle detection, deterministic ordering, error collection, and response-size
  limits.
- Have remote filesystems use the server's walk operation directly.

## Testing

- Cover local and remote handling of invalid roots and limits, directory
  symlinks, non-UTF-8 names, cancellation, sandbox contexts, and response
  budgets.

GitOrigin-RevId: 7499bf05080c3f9965a5eb7ffd593de604d62c2a

codex-rs/ext/skills/tests/executor_file_system_authority.rs

@@ -25,6 +25,10 @@ use codex_exec_server::GetMetadataOptions;
 use codex_exec_server::ReadDirectoryEntry;
 use codex_exec_server::ReadFileOptions;
 use codex_exec_server::RemoveOptions;
+use codex_exec_server::WalkEntry;
+use codex_exec_server::WalkEntryKind;
+use codex_exec_server::WalkOptions;
+use codex_exec_server::WalkOutcome;
 use codex_exec_server::WriteFileOptions;
 use codex_protocol::capabilities::CapabilityRootLocation;
 use codex_protocol::capabilities::SelectedCapabilityRoot;
@@ -191,6 +195,34 @@ impl ExecutorFileSystem for SyntheticFileSystem {
         Box::pin(SyntheticFileSystem::read_directory(self, path))
     }
 
+    fn walk<'a>(
+        &'a self,
+        path: &'a PathUri,
+        options: WalkOptions,
+        _sandbox: Option<&'a FileSystemSandboxContext>,
+    ) -> ExecutorFileSystemFuture<'a, WalkOutcome> {
+        Box::pin(async move {
+            self.metadata(path)?;
+            assert_eq!(path, &self.canonical_root);
+            assert!(options.max_depth >= 1);
+            assert!(options.max_directories >= 2);
+            assert!(options.max_entries >= 2);
+            Ok(WalkOutcome {
+                entries: vec![
+                    WalkEntry {
+                        path: self.path("skill")?,
+                        kind: WalkEntryKind::Directory,
+                    },
+                    WalkEntry {
+                        path: self.path("skill/SKILL.md")?,
+                        kind: WalkEntryKind::File,
+                    },
+                ],
+                ..WalkOutcome::default()
+            })
+        })
+    }
+
     fn remove<'a>(
         &'a self,
         _path: &'a PathUri,